Ground Reaction Force (GRF) is defined as a force vector applied by the ground to a person at a point on a person's footprint called the Center of Pressure (COP). The GRF direction can be modeled by a force vector colinear with a line connecting ankle and hip. Neglecting air friction, the average horizontal component of the GRF must exactly equal zero for a person walking/running at constant average velocity regardless of ground slope. If this were not the case, then a person's torso would increase or decrease its average horizontal velocity. Similarly, the average vertical component of the GRF must be exactly equal to body weight regardless of the ground slope. If this were not the case, then the average distance from the torso to the ground would increase or decrease.
The function of the legs in human bipedal locomotion (hereafter locomotion) is to make periodic ground contact with a foot during each step for the purpose of transferring the GRF to the torso. By making the average horizontal GRF less than or greater than zero, the torso can be accelerated or decelerated. The locomotion process is one whereby the torso weight is supported alternately between one leg and the other. Each step consists of a support epoch during which one leg supports the torso weight while the alternate leg is swinging forward preparing for the next step. Each support epoch is followed by a transition epoch where the torso weight transitions between the current support leg to the new support leg.
Others have created locomotion assist devices throughout recorded history mostly with the intention of mitigating leg dysfunction. Locomotion assist devices are currently in widespread use today in the form of crutches, canes, and a variety of knee braces. Some locomotion assist devices include wheeled devices, such as bicycles, wheelchairs, scooters, and other alternatives to human bipedal locomotion. These devices transfer the GRF to the torso from points on the ground which are constantly moving.
Canes and crutches are devices that allow a user to transfer a portion (up to all) of the GRF to the torso via the arm sockets. These devices are effective in reducing the gait problems caused by one or both dysfunctional legs and are commonly employed. Unfortunately these devices are problematic for long term use, over uneven terrain, or where the arm sockets are not suitable for transferring substantial portions of the GRT to the torso.
Locking knee braces are another class of device commonly used to transfer the GRF to the torso. Unlike canes and crutches, locking knee braces transfer the GRF to the torso via the hip sockets. Since this is the normal mechanism for humans to transfer GRF to the torso, it is a preferred mechanism. Common locking knee braces consist of a shank and thigh frame coupled together with a hinge that can be locked at an explicit knee angle when torso support is required. Knee braces are widely used to reduce knee joint stresses and provide knee immobilization following surgery. In general they are not employed as walking enhancement devices because the fixed knee angle greatly impedes normal action of the leg during walking/running.
There exists several intelligent, electronic knee braces used to control resistive torque or damping about the knee joint. These knee braces are primarily intended to mitigate leg dysfunction caused by amputation. Using sensory information, these active braces can discriminate between early and late support phases thereby allowing amputees to flex their knee just after heel strike. This feature is important for shock absorption and is not possible with prior mechanically passive prosthesis. Electronic knees can also supply different levels of damping during swing and support dependent on walking speeds using adaptive algorithms. Several of these electronically controlled knee braces have been commercialized, such as the Otto Bock's C-leg and Ossur's Rheo Knee.
U.S. Pat. Nos. 6,500,138, 6,834,752, and U.S. Patent Publication 2003/0062241 disclose a knee brace which provides support while allowing unimpeded knee angle flexion during leg swing. This device, called an auto-locking knee brace, employs a microprocessor controlled one-way clutch at the knee joint of a common knee-ankle-foot-orthotic. This device has two modes of operation. In one mode, the one-way clutch is inactivated thereby allowing free rotation of the foot/shank and thigh frames. One-way clutches have a well known property that when activated they have an easy rotation direction where only a small amount of torque can be coupled from input to output and a hard direction where an arbitrarily large amount of torque can be coupled from input to output. This feature of the one-way clutch is exploited in the auto-locking brace to allow relatively unimpeded leg extension prior to heel strike while providing full support following heel strike. Activation of the one-way clutch utilizes a solenoid. For polio and stroke patients, the auto-locking knee has shown significant improvements including reducing metabolic energy consumption of wearers of the device.
Numerous mobility assist devices have been developed over the years employing mechanisms which incorporate means for temporarily storing and releasing the energy generated and needed during each step. U.S. Pat. Nos. 420,178 and 420,179 disclosed a device employing bow springs attached to a shoulder and a pelvis. The '179 Patent incorporates a foot-lift mechanism to enable swing leg foot clearance, however does not teach a workable mechanism for activating the foot-lift mechanism.
U.S. Pat. No. 4,872,665 discloses a running brace that employs a telescoping gas spring and a swing leg foot clearance mechanism employing a ratchet joint. The disclosure does not discuss practical methods for release of the ratchet joint. The primary problem with the device is there is no mechanism for controlling the natural release of the energy stored in the gas spring phase locked to the gait cycle selected by the wearer. In particular, assuming that an embodiment of the device is possible, a wearer must adjust his gait cycle to the natural frequency dictated by the physical parameters of the device.
U.S. Pat. No. 5,016,869 discloses a running assist device for enhanced mobility and reduced metabolic energy consumption. In this device, the GRF is coupled to the torso mass directly through the legs without any mechanism working in parallel with the legs. Accordingly, the device cannot store energy available during the period of time when the distance between hip socket and ankle is decreasing. In effect, the device acts as a mechanism for transferring GRF to the soles of the wearer's feet, not the torso.
U.S. Pat. Nos. 4,967,734, 5,011,136, and U.S. Patent Publication 2002/0094919, disclose energy-efficient running braces employing a mechanical spring which temporarily stores the energy during the period when the distance between torso center of mass and the ground is decreasing and releases the energy during the period when that distance is increasing. These devices support the torso via a torso harness and refer to means for generating a constant leg thrust. Measurement of the GRF reveal that the required leg thrust increases substantially as running speed is increased, reaching up to five times body weight during sprinting. Benefits of these devices do not appear to be achievable by a wearer suffering single leg dysfunction. The devices also appear to require extensive periods of time to doff and don. All of the above-mentioned designs couple the GRF to the torso via a torso harness.